Remote Login And Desktop

TELNET, SSL

Remote Interactive Computing• TCP make possible interactive use of remote

machine.• A client program establishes a TCP connection

to server, then start sending commands to the server and displays output send by the server.

• Problem: One server for each computation service will overload the machine with server processes.

• Solution: Create a machine that allows a user to establish a session on the remote machine and then run any application. This is known as Remote Desktop Facility.

TELNET Protocol• TELNET is the TCP/IP protocol suite member.

It’s a simple textual remote terminal protocol.• TELNET establishes a TCP connection and

passes keystrokes from the user’s keyboard directly to the remote computer.

• TELNET also carries textual output from the remote machine back to the user’s screen.

• This service is called transparent.

Basic services provided by TELNET• It defines a network virtual terminal, that

provides a standard interface to remote systems.

• It includes a mechanism that allows the client and server to negotiate options, and provides a set of standard options.

• TELNET treats both ends of the connection symmetrically. It does not force client input to come from a keyboard nor the client has to display the output on the screen.

User’s Keyboard & Display

Operating System

TELNET Client

Client reads from terminal

TCP / IP Internet

Operating System

TELNET Server

Client sends to server

Server receives from client

Server sends to pseudo terminal

Advantages and Disadvantages for the TELNET server to be an application level program

• Advantage : It makes modification and control of the server easier than if the code were embedded in the OS.

• Disadvantage : Inefficiency

Accommodating Heterogeneity• To make TELNET interoperate between

as many as systems possible, it must accommodate the details of various computers and OS.

• The specific keystrokes used to interrupt a program varies from system to system.

• TELNET defines how data and command sequences are sent across the internet, is known as the Network Virtual Terminal (NVT).

Definition of NVT format

• All communication invokes 8 bit bytes.

• It uses the standard 7-bit USASCII representation for data and reserves byte with the higher order bit set for command sequences.

• All characters that represent printable values are assigned the same meaning as in the standard USASCII character set.

ASCII

Control Code

Decimal Value

Assigned Meaning

NUL 0 Has no operation, no effect on o/p.

BEL 7 Sound audible/visible signal.

BS 8 Move left one character position

HT 9 Move right to the next horizontal tab

LF 10 Move down to the next line.

VT 11 Move down to the next vertical tab.

FF 12 Move to the top of the next page

CR 13 Move to the left margin on the current line.

Other Control - Has no operation, no effect on o/p.

Passing commands that control the Remote side• TELNET NVT accommodates control functions by

defining how they are passed from the client to the server. eg. CTRL + C in Unix will terminate the running program.

Signal Meaning

IP Interrupt Process (Terminate Running Program)

AO Abort Output (Discard any buffered output)

AYT Are You There (test if server is responding)

EC Erase Character (delete the previous character)

EL Erase Line (delete the entire current line)

SYNCH Synchronize (clear data path until TCP urgent data point, but do interrupt commands)

BRK Break (break key or attention signal)

Forcing the server to read a control function

• PROBLEM:• Sending control functions along with normal data is

not always sufficient to guarantee the desired results.

• If the application at the server’s side stops reading i/p, OS buffer will eventually fill and the server will block attempting to write data to the pseudo terminal.

• If the user generates an interrupt control function when buffers are filled, the control function will never reach the server.

Solution:

• TELNET uses an out of band signal.

• It implements out of band signaling with the urgent data mechanism.

• Whenever it places a control function in the data stream, also sends SYNCH command.

• TELNET appends a reserved octet called the Data Mark, and causes TCP to signal the server by sending a significant URGENT DATA bit set.

Telnet Options

• Whether the TELNET operates in Half or Full Duplex mode.

• Allow the server on a remote machine to determine the user terminal type.

The terminal type is important for software that generates cursor positioning sequence.

Examples of Telnet OptionsName Code RFC Meaning

Transmit Binary

0 856 Change transmission to 8-bit binary

Echo 1 857 Allow one side to echo data it receives

Suppress GA 3 858 Suppress Go-Ahead signal after data

Status 5 859 Request for status of a TELNET option from remote side.

Timing –Mark 6 860 Request timing mark inserted in return stre- am to synchronize two ends of connection.

Terminal Type

24 884 Exchange information about the make and model of the terminal being used.

End of Record

25 885 Terminate data sent by EOR code.

Line Mode 34 1116 Send complete lines instead of individual characters.

Option Negotiation

• TELNET uses a symmetric option negotiation mechanism to allow clients and servers to reconfigure the parameters, controlling their interaction.

• Because all TELNET software understand a basic NVT protocol, clients and servers can interoperate even if one understands options another does not.

• In telnet terminology : Request is WILL X

Response is either DO X or DON’T X

Secure Shell (SSH)

• Popular alternative to TELNET.• Provides 2 significant enhancements:

1) Provides secure connection.

2) Provides users with the ability to perform additional, independent data transfer over the same connection that is used for remote login.

• SSH offers 3 mechanisms that form basis of the services.

1. A Transport Level Protocol provides server authentication, data confidentiality and data integrity with perfect forward secrecy.

2. A user authentication protocol that authenticates the user to the server. Thus, a server can tell exactly which user is attempting to form a connection.

3. A connection protocol that multiplexes multiple logical communication channels over a single underlying SSH connection.

• SSH uses public key cryptography for server authentication and allows the use of either iterative passwords or public key cryptography for user authentication.

• Secret Key/ Session key is negotiated by the client and the server before any application data transfer occures.

Port Forwarding

• Most powerful aspect of the SSH multiplexing mechanism, is port forwarding

• SSH connection can be used as a secure tunnel between two computers, and a user can configure SSH to automatically splice an incoming TCP connection to a new connection across the tunnel.

• Splicing that occurs when a connection is made to the client side of the tunnel is known as Local Port Forwarding.

• Splicing that occurs when a connection is made to the server side is known as Remote Port Forwarding.

Advantage of Port Forwarding

• It allows arbitrary applications to pass data between two sites – instead of having two separate client and server software implement encryption for each application, SSH can be configured to allow all of a user’s applications to communicate over a single SSH connection.

Other Remote Access Technologies1. Rlogin : Earliest alternative to TELNET, part of BSD Unix

OS. It created an idea of trusted hosts. User can control access to their personal account by

authorizing remote login based on remote host and remote user names.

2. Virtual Network Computing (VNC) : Provides a remote desktop capability instead of a textual interface. Runs across multiple platforms.

3. Remote Desktop Protocol (RDP): Microsoft has defined RDP for use with their OS. It works like other remote desktop system.